In the cultivations of crops, fruit, and the like, plants should be supplied with water and nutrients in terms of productivity at appropriate times based on the growing status of the plants. Thus, grasping the growing status of the plants properly without affecting growing of the plants is essentially important.
In many actual situations in the present agricultural fields, the growing status of plants is grasped according to human experience based on the number of days without rain or by intuition, for example. However, managing the growing status of plants by a method based on experience, and the like is skillful work that involves much expense in time and effort. Additionally, barometers as a reference in such management are determined from personal experiences, for example. Hence, not everyone finds it easy to implement such a method of grasping the growing status of plants based on experience, etc.
On the other hand, various techniques have been developed in recent years intended to execute water control or fertilization management of crops or fruit based on biological information of a plant. Among these techniques, a measuring method using the Granier method is a notable method. This method is to enable grasp of biological information of a plant more correctly as it allows a sap flow of the plant to be measured directly (see patent literature 1, for example).
Patent literature 1 discloses a device with a rod-shaped temperature sensor and a rod-shaped heater-equipped sensor that can be arranged in holes formed in the trunk of a tree with a drill, for example. According to a technique disclosed in patent literature 1, both of these sensors of the device are arranged in the holes formed in a sapwood part of a tree. After elapse of a predetermined time, the flow rate of sap flowing in the tree is determined based on a temperature difference between these sensors.
The device of patent literature 1 has originally been developed for measurement of a tree having a large stem diameter (more specifically, a stem diameter of 20 cm or more). The rod-shaped sensors used in the device are formed into a size from 2 or 3 mm or more in diameter and from 2 to 3 cm or more in length. Thus, the device of patent literature 1 is not applicable to a plant having a stem diameter of less than 20 cm. Additionally, for measurement of a sap flow rate, this device requires formation of a hole in a tree with a drill, for example. Hence, a sap flow rate cannot be measured using the device of patent literature 1 unless a few days have elapsed after installation of the device. Further, chipping off the epidermis of a tree, forming holes in the tree with a drill, and inserting the sensors is destructive testing.
A device also applicable to a plant having a small stem diameter has been developed (see patent literature 2). Patent literature 2 discloses a stem liquid flow measurement sensor formed of a thin-film base rectangular in a plan view and having a longitudinal length (lengthwise direction) from about 15 to 20 mm, a crosswise length of about 10 mm, and a thickness from about several hundreds of micrometers to 1 mm. This stem liquid flow measurement sensor includes a thin-film temperature-measurement metallic resistive elements in a pair formed on the base and a thin-film metallic heater provided between these resistive elements in a pair. According to the recitation of patent literature 2, the stem liquid flow measurement sensor is inserted to about two-thirds from its surface in the lengthwise direction into a cut formed in the axis direction of the stem of a tomato. Then, this sensor is arranged in such a manner that the resistive elements and a heater are arranged in a xylem inside the stem. In this way, a stem liquid flow can be measured while adverse effect on a plant is reduced.